Direct line-of-sight optical communications has a long history. The use of directed beams, such as lasers for this purpose is the latest incarnation of this technology. It has become known as optical wireless (OW) or free-space optical (FSO) communications. Although OW test systems of this sort were developed in the 1960's, the technology was not widely accepted. Optical fiber communications had not been developed, and a need for a high-bandwidth bridging technology did not exist. The proliferation of high-speed optical fiber networks has recently created the need for a high-speed bridging technology that will connect users to the fiber network, since most users do not have their own fiber connection. This has commonly come to be known as the “first” or “last” mile problem.
Radio frequency (RF) wireless systems may be used as a solution to the bridging problem, however they are limited in data rate due to the low carrier frequencies involved. In addition, because “broadcast” technology is generally regulated, it must be operated within allocated regions of the electromagnetic spectrum. Spread-spectrum RF, especially emerging ultra-wideband (UWB) technology, is able to avoid spectrum allocations provided transmit powers are kept low. However, this generally limits the range to a few tens of meters.
With the implementation of dense wavelength-division multiplexing (DWDM), the information-carrying capability of fiber optic networks has increased enormously. At least 10 Tb/s of capacity on a single fiber has been demonstrated. This capacity would, in principle, allow the simultaneous allocation of 10 Mb/s each to one million subscribers on a single fiber backbone. The problem is, however, to provide these capacities to actual subscribers, who in general do not have direct fiber access to the network. Currently, the maximum that is available to most consumers is wired access to the network, since fiber comes to the telephone company's switching stations in urban or suburban areas, but the consumer has to make the connection to the station. Clever utilization of twisted-pair wiring has given some consumers network access at rates from 128 kb/s to 10 Mb/s, although at most access of this kind through digital subscriber lines (DSL) is limited to about 1.5 Mb/s. Cable modems are able to provide access at rates of about 30 Mb/s, however multiple subscribers must share a cable, and simultaneous usage by more than a few subscribers drastically reduces the data rates available to each. The bridging problem possibly may be solved by laying optical fiber to each subscriber, yet this would be without the demand for the service from enough subscribers, the various communications service providers are generally unwilling to commit to the investment involved which is estimated at $1,000.00 per household.
Optical wireless provides an attractive solution to the first-mile problem, especially in densely populated urban areas. Optical wireless service may be provided on a demand basis without the extensive prior construction of an expensive infrastructure. Optical transceivers can be installed in the windows or on the rooftops of buildings and communicate with a local communication node, which provides independent optical feeds to each subscriber. In this manner, only paying subscribers receive the service. The distance from individual subscribers to their local node would generally be kept below 300 meters, and in many cases, in cities with many high-rise apartments, this distance will be less than 100 meters. These distances are kept small to insure reliability of the optical connection between subscriber and node.
Deployment of optical wireless network architectures and technologies as extensions to the internet is contingent on the assurance that their dynamic underlying topologies are controllable with insured and flexible access. In addition, this wireless extension must provide compatibility with broadband wire line networks in order to meet requirements for transmission and management of terabytes of data.
The RF spectrum is becoming increasingly crowded and demand for available bandwidth is growing rapidly. As a result, OW communication as a primary means for communication is becoming more appealing. At the low carrier frequencies of RF communication, even with new bandwidth allocations in the several gigahertz region, individual subscribers can obtain only modest bandwidths, especially in dense urban areas. Since conventional wireless is a broadcast technology, all subscribers within a cell must share the available bandwidth, cells must be made smaller, and their base station powers must be limited to allow spectrum re-use in adjacent cells.
Recent research has shown that multi-hop, RF wireless networks, i.e., RF peer-to-peer or “infrastructure-less” networks are generally not scalable, and the size and number of users is limited. Optical wireless provides an attractive way to circumvent such limitations. This line-of-sight communication technology avoids the wasteful use of both the frequency and spatial domains inherent in broadcast technologies. Optical wireless provides a secure, high data rate channel exclusively for exchanging information between two connected parties. There is no spectrum allocation involved since there is no significant interference between different channels, even between those using the exact same carrier frequency. Thus, this technology provides a secure communication network for critical applications, such as on a field of battle during a war conflict.
A fundamental difficulty arises in the use of directed beams to communicate information between opposing nodes of a communication network in that the communication between nodes is terminated if the directed beam is obscured or obstructed. Thus, there is an apparent need for a method to reroute communications in such an event, while maintaining the integrity of the network with a minimal amount of interruption of service.